Serving for registering process variables in automation technology are, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, analytical measuring devices, etc., which register corresponding process variables, fill level, flow, pressure, temperature, analytical data, such as pH-value, turbidity or conductivity. Measuring devices are composed essentially of a sensor element, which delivers information concerning the process variable, and at least one electronics unit, which activates the sensor element, conditions and/or evaluates information delivered by the sensor element and provides measured values of the process variable.
Measuring devices are applied in an industrial environment, frequently in a process environment, whose temperature lies significantly above the maximum allowable temperature of the temperature sensitive components of the sensor element or the electronics unit. In order to avoid destruction of a temperature sensitive component, which usually leads to the failure of the measuring device, there is provided, for example, between the sensor element, which with the process, and the electronics unit having the temperature sensitive component, a connecting component, whose thermal resistance is sufficiently high that the sensor element and the electronics unit are thermally decoupled from one another to the required degree.
Moreover, measuring devices in the case of use in the chemical or pharmaceutical industry, however, also in the foods field, are, e.g. due to cleaning processes, often subjected to rapid temperature changes sequentially following one another. As a result of rapid temperature changes, high temperature gradients are present, at least for a short-time. These temperature gradients only disappear after thermal equilibrium has been achieved between measuring device and process. Since measuring device are manufactured of different materials with different coefficients of expansion, mechanical stresses occur in the measuring device. In the worst case, temperature fluctuations or temperature gradients lead to fracture or gap formation on or in the measuring device.
Due to different design conditions, such as e.g. required pressure resistance and/or electrical conductivity, it is often necessary in the case of industrial applications to manufacture the thermally decoupling connecting component of a material, which has the properties of metal as regards strength and conductivity. The usually high thermal conductivity of metals opposes, in principle, the desired thermal decoupling. In the state of the art, it is attempted to achieve a high thermal resistance and therewith a good thermal decoupling via the geometry of the connecting component. Especially, a desired thermal resistance can be implemented by a suitable cross-section reduction and/or a suitable increasing of the length of the connecting component.
Disadvantageous in the case of known solutions is that it is difficult to achieve a compact embodiment of a measuring device, when for the purpose of thermal decoupling a connecting component with increased length is used. Also, a cross-section reduction has its limits, since, below a certain cross section of the connecting component, the mechanical stability required at the industrial location of use of the measuring device is no longer assured.
Known from German patent, DE 10 2009 028 620 A1 is a measuring device for monitoring the fill level of a fill substance in a container. This measuring device is so embodied that it is suitable for a high temperature process. The high temperature range is specified in connection with the invention as temperatures above 150° C. The measuring device is composed of a sensor element located in the process and a measurement transmitter located outside of the container.
The embodiment of the temperature reduction unit 18 shown in FIG. 1 is based on FIG. 3 of DE 10 2009 028 620 A1. Via the coupling unit 17, the high-frequency measurement signals are coupled into a hollow conductor 20, respectively coupled out of the hollow conductor 20. The measurement signals are generated in a signal producing unit (not shown in FIG. 1). Arranged in the hollow conductor 20 is a dielectric process isolator 19, which prevents that particles from the process migrate into the electronics unit (not shown). The process isolation 19 is so embodied that the electromagnetic measurement signals guided in the hollow conductor 20 are influenced as a little as a possible.
High thermal resistance is achieved at the temperature reduction unit 18 of the known solution via a slimming of the diameter. Additionally, the temperature reduction unit 18 includes cooling fins. Temperature reduction unit 18 is so embodied that the required temperature reduction between the temperature in the process and the ambient temperature in the space outside of the container where the process transpires, is assured.
Known from German patent, DE 1012 103 493 A1 is another embodiment of a fill-level measuring device suitable for high temperature applications. High-frequency measurement signals with a frequency greater than or essentially equal to 26 GHz are transmitted via an antenna unit in the direction of the surface of a fill substance located in a container, and the echo signals reflected on the surface of the fill substance are received via the antenna unit. In a fill substance facing end region of the hollow conductor guiding the high-frequency measurement signals, a first, gas-sealed, process isolation is provided. This is at least partially manufactured of a ceramic material and is so embodied that it passes the measurement signals almost reflection freely. Further provided in the hollow conductor is a second gas tight, ceramic process isolation, which is so embodied that it passes the high-frequency measurement signals, on the one hand, almost reflection freely, and, on the other hand, withstands a higher mechanical load than the first process isolation. A temperature reduction unit surrounds a portion of the hollow conductor radially and is arranged in a region between the second process isolation and the control/evaluation unit. Also here, the temperature reduction unit is so embodied that the temperature sensitive components in the control/evaluation unit are exposed to temperatures, which lie within their working range.
In order purposefully to counteract the high temperature gradients, which occur before reaching thermal equilibrium, sensitive components in the state of the art are—such as already stated—thermally decoupled from the insensitive measuring device components. A suitable thermal coupling is achieved via a corresponding adapting of the geometry of the connecting component. Further known for thermal decoupling are also double walled structures with an enclosed, thermally insulating, air layer. Corresponding insulating connecting components are, however, relatively complex to manufacture and, consequently, correspondingly expensive.